During a first phase of the development of synthetic fibres made of polyethylene terephthalate (termed PET herein for short) such fibres were produced in a two-stage process. In the first stage, substantially amorphous fibres were made by melt spinning, and were fully stretched in the second stage on a separate machine, cf. Marshall et al in J. Appl. Chem, 4 (1954), pages 145-153.
Even during that stage, heating of the filaments by contact with one or more solid surfaces was believed to be essential for maximum stretching and, hence, molecular orientation and high tensile strength. Apparently, it is the relative rigidity of the polymer chain of PET caused by the phenylene moieties that is at the root of the problems connected with molecular orientation of PET filaments upon stretching.
Upon further development of this technology, pre-oriented yarns (POY) were produced in the first stage but full stretching still required a second stage and a separate machine. More recently, use of superheated steam has been suggested as a means for improving yarn drawing technology (e.g. as reported by Ethridge F.A. et al; IFJ June 1989, pages 64-68.).
While PET yarn production has been and important field of technology of man-made fibres ever since PET became available in commercial quantities, the importance has increased substantially with the advent of PET as a replacement for glass bottles and the consequent problems of recycling ever increasing quantities of PET scrap. While PET in virgin state already tends to cause processing problems, use of feed materials containing, or even consisting of, PET scrap and subsequent problems of thermally degraded polymer materials seemed to indicate that melt spinning was-not a promising avenue for re-use of PET. On the other hand, polymer fibre production methods tend to generate more valuable products than most other methods of polymer processing.
Accordingly, it is a main object of the invention to provide for a method of producing PET yarns even when the feed stock contains substantial amounts of recycled PET.
Other objects will become apparent as this specification proceeds.